Not applicable
1. Field of the Invention
The present invention relates to friction welding. More particularly, the present invention provides an improved method and apparatus that relates to friction plug welding suitable for flight hardware usage, wherein the pull plug has an enlarged heat with a chamfered heat sink.
2. General Background of the Invention
Friction stir welding (FSW) is a solid state joining process developed by The Welding Institute (TWI), Cambridge, England and described in U.S. Pat. No. 5,460,317, incorporated herein by reference. Also incorporated herein by reference are U.S. Pat. No. 5,718,366 and all references disclosed therein.
The following references are also incorporated herein by reference: U.S. Pat. Nos. 3,853,258, 3,495,321, 3,234,643, 4,087,038, 3,973,715, 3,848,389; British Patent Specification No. 575,556; SU Patent No. 660,801; German Patent No. 447,084, xe2x80x9cNew Process to Cut Underwater Repair Costsxe2x80x9d, TWI Connect, No.29, January 1992; xe2x80x9cInnovator""s Notebookxe2x80x9d, Eureka Transfer Technology, October 1991, p. 13; xe2x80x9cRepairing Welds With Friction-Bonded Plugsxe2x80x9d, NASA Tech. Briefs, September 1996, p. 95; xe2x80x9cRepairing Welds With Friction-Bonded Plugsxe2x80x9d, Technical Support Package, NASA Tech. Briefs, MFS-30102; xe2x80x9c2195 Aluminum-Copper-Lithium Friction Plug Welding Developmentxe2x80x9d, AeroMat ""97 Abstract; xe2x80x9cWelding, Brazing and Solderingxe2x80x9d, Friction welding section: xe2x80x9cJoint Designxe2x80x9d, xe2x80x9cConical Jointsxe2x80x9d, Metals Handbook: Ninth Edition, Vol. 6, p. 726.
Friction plug welding (FPW), also referred to as plug welding and friction taper plug welding (FTPW), is a process in which initial defective weld material is located, removed and replaced by a tapered plug, which is friction welded into place. This process is similar to friction stud welding, in which a plug is welded to the surface of a plate, end of a rod, or other material. The primary difference is that FPW is designed to replace a relatively large volume of material containing a defect whereas friction stud welding is a surface-joining technique.
Friction plug welding could be used to repair weld defects in a wide variety of applications; however, it would most likely be used where weld strength is critical. This is due to the fact that manual weld repairs result in strengths much lower than original weld strengths, as opposed to friction plug welds (FPWs) whose typical mechanical properties exceed that of the initial weld. In applications where high strength is not required, manual welding would be less expensive and would not require specialized equipment.
An extension of FPW is known as stitch welding or friction tapered stitch welding (FTSW) and has been developed to repair defects longer than what a single plug can eliminate. Stitch welding is the linear sequential welding of several plugs such that the last plug weld partially overlaps the previous plug. Defects of indefinite length can be repaired with this process, limited only to the time and cost of performing multiple plug welds. These welds have undergone the same testing procedures as single FPWs, including NDI and destructive evaluation. The strengths for stitch welds are similar to those for single plug welds.
Stagger stitch welding is a process best defined as stitch welding in a non-linear fashion. Areas wider than one plug length can be completely covered by staggering plugs side to side as they progress down the length of an initial weld. This process is being developed for plug welds whose minor diameter is on the crown side of the initial weld, and where replacement of the entire initial weld is desired.
While friction plug welding might be a preferred method of repairing defects or strengthening initial welds, there are some applications where heretofore it has been extremely difficult to use friction plug welding. The main cause is due to the logistics of setting up the equipment and/or support tooling to perform friction plug welding, and the geometry of the work piece to be welded.
The apparatus of the present invention solves the problems confronted in the art in a simple and straightforward manner. What is provided is an apparatus for and a method of friction plug welding an article using a plug that has a geometry that provides less heat sink and facilitates a good weld when the plug is pulled. As demonstrated through several single variable experiments, a larger plug mass acting as a heat sink at the top of the weld has deleterious affects on the bonding of the plug top. Thermodynamically, plug mass acts to conduct heat away from the interface, while the atmosphere insulates the interface, increasing the heating capabilities of the welding process. With less plug mass, or heat sink, left on top of the plate surface after the completion of a weld, the heat produced by the weld process is not conducted away from the interface as rapidly.
Due to the nature of the Friction Pull Plug Welding process, xe2x80x9ccoldxe2x80x9d plug material is always being pulled into a xe2x80x9chotxe2x80x9d interface, with the plug shaft being the first location of the plug to be heated. This situation poses a problem to ensuring complete plug/plate interfacial bonding at the top of the weld, the last location of bonding during the welding process. Since the lack of bonding defect at the topside of the weld is easily detected through dye-penetrant non-destructive inspection, it has been thoroughly characterized and analyzed. The problem of bonding this last interfacial location is quite complex, involving a combination of both loading and heat flow.
Finite Element Thermal Modeling has demonstrated the validity of the above thesis of heat flow within the pull plug. Two plug models have been described by the thermal model, Model 1 with 0.500 inches and Model 4 with 0.100 inches of plug mass left above the plate surface after welding. The divergence of the temperature versus distance (distance from the bottom of the plate to the top of the heat sink) curves at the region of final bonding, exhibits the effects of heat conduction. The plug with less heat sink, maintains a higher temperature at the plug top, due to atmospheric insulation and less heat conduction through plug. At a radial distance of 0.100 to 0.200 inches from the plug interface, the divergence of the curves reaches a maximum, indicating the overall importance of plug mass heat sink at this location.
Thermal modeling and experimental testing have both demonstrated the need to minimize the plug mass heat sink remaining above the plate surface to ensure complete interfacial bonding at the plug top. However, three major problems can arise from a minimized heat sink, the entire plug could be pulled through the plate hole, the central portion of the plug could be separated along grain boundaries, or with a top hat plug, the plug top hat can be separated from the body. Each of these conditions is completely unacceptable for a production acceptable process.
The forging load can completely pull a low angle plug through the plate hole because of plug overheating (i.e. does not have enough mass heat sink) or because of too little enough plug mass resistance (i.e. no top hat). A central pull-through can occur in higher angle plugs without a substantial heat sink, where the angle of the plug prevents complete plug pull through. In high angle plugs, the loading is transferred to the high heat region at a radial distance of 0.100 to 0.200 inches from the interface. The heated plug grain boundaries do not have adequate strength to support the forging load and a central plug pull-through occurs. A similar situation arises during a top hat separation, where the interfacial bonding and the geometry of the top hat, act to transfer loading to the temperature weakened plug material at the hat/plug body transition region. The forging load overcomes the shear strength of the heated grain boundaries leading to a top hat separation, which is usually followed by central plug pull-through.
The present invention includes a method of friction plug welding an article, comprising several stages. Preferably, the first stage is making a hole (that is preferably tapered) in the article to be welded. Machining a tapered hole is not necessarily required in friction plug push welding where (in certain situations generally characterized when the article to be welded is softer (having lower hardness) relative to the harder (having higher hardness) plug) the plug will form a hole, self bore or embed into the material either while rotating or not. In copending application Ser. No. 60/156,734, filed 09/30/99 and entitled xe2x80x9cFriction Pull Plug Welding: Dual Chamfered Plate Holexe2x80x9d, incorporated herein by reference, there is disclosed chamfered hole geometry for use in friction plug welding. A tapered plug is then inserted through the tapered hole, then the plug is attached to a chuck of a rotary tool or like motor which can both pull on the tapered plug and rotate it. Some connection means, such as threads, key grooves, flats, or locking retention interface, are provided on the tapered plug to facilitate pulling the plug with the rotary tool.
The second stage, or heating cycle is always required to weld the plug to the article. This stage preferably consists of rotating the plug while pulling (placing the plug in tension axially) into intimate contact with the hole""s surface, or region surrounding the hole. The typical axial load exerted on the plug during the heating phase is between about 1000 pounds and 20,000 pounds, preferably between about 6000 pounds and 18,000 pounds, more preferably 10,000 pounds to 16,000 pounds, and most preferably 12,500 pounds to 15,000 pounds.
Other forms of heating may also be utilized, including but not limited to, using electricity to assist in the heating process, or vibrational energy such as oscillatory rotation rather than the preferred method of continuous rotation, or lateral, axial or some combination thereof, rapid displacement (such as ultrasonic welding) to impart sufficient energy to assist in the heating the weldment. The plug (preferably tapered, with a taper the same as or preferably different from the taper of the hole (if it is tapered), and rotating the plug relative to the part while moving the plug in the direction such to make contact with the hole""s surface, until contact is made, and forcing the plug into the surface of the hole by pulling on the plug (imposing a tensile force in the plug in the plug""s axial direction) all while continuously spinning the plug relative to the article.
The third stage is the braking stage. This rapid deceleration of rotation, if rotation is used, or otherwise defined as rapid decline of energy input to zero or near zero, is necessary to performing a successful weld. Preferably, the fourth stage which is also referred to as the forging stage, is a period of cooling in which no further heating energy is intentionally applied to the weldment and energy in the form of heat is dissipated. During this stage, it is preferable to maintain either the same axial tensile load, or a different axial tensile load whether that be greater or lesser, to cause densification and or maintain or create a sound metallurgical bond or weldment. In the current application, although not necessarily required in other applications, excess sections of the plug are cut off and material further removed via grinding and sanding to make it smooth with the initial weldment and/or surrounding materials"" surfaces. The present invention also includes the plug and its upper end with chamfered heat sink.
The displacement during heating should be optimized for the specific plug geometry and hole geometry combination being welded. Empirical models can be developed to ensure that the heating displacement is great enough to enact the benefits of the xe2x80x9ctop hatxe2x80x9d, while not producing a weld with defects, such as weld pull through, lack of bonding, or grain separation.
In the preferred embodiment of the method of the present invention, a tapered hole is drilled from one side of the article being repaired. A tapered plug is then inserted through the tapered hole, then the plug is attached to a chuck of a motor which can both pull on the tapered plug and spin it. Some connection means, such as threads or locking retention interface, are provided on the tapered plug to facilitate pulling the plug.
The plug is pulled while spun by the motor. Preferably the plug is pulled also after the spinning stops, with a load the same as or different from the load while spinning. After the spinning has taken place and the plug is welded in place, the excess part of the plug is cut off and the weld machined down to make it smooth. Pulling a tapered plug during plug welding allows all equipment, including a backing plate, to be on one side of the article being welded. Pull welding eliminates the need for large backing structures that must react high loads associated with friction plug push welding, often exceeding 10,000 pounds force, while at the same loads deflect an amount often less than 0.25 inches. A hydraulically powered direct drive weld has been used; however, an electrically powered direct drive, or inertia drive flywheel weld system may also be used.
The inventors have found that satisfactory welds occur most frequently when the plug diameter is large enough to maintain a mechanically stable cool core. For this reason, plug diameters have continued to increase, and more powerful weld equipment has been acquired. Techniques have been developed to weld larger diameter plugs while minimizing the required motor power. One such finding entails varying the axial stroke rate during the weld process to decrease the initial contact friction. In this process, it is preferable for the plug and article to contact slowly, thereby reducing the rotational friction at contact. After the boundary between the plug and article plasticizes, then it is preferable, although not required, to increase the stroke rate, thereby increasing the rate of heating at the interface to achieve weld temperatures. This discovery significantly reduces the required power to perform welds, and is advantageous in performing large welds whose power requirement exceeds that which the system is designed to deliver.
The inventors have found that with their current equipment and process, the preferable operating range at which to rotate the plug is 4000-6000 rpm prior to contact between the plug and hole""s surface, and it is also preferable to maintain a minimum of 3000 rpm during the duration of the heating cycle. Successful welds have been created at much slower speeds, as low as but not limited to 1000 rpm prior to contact and as high as, but limited only by the equipment capability, of about 7000 rpm prior to contact.
The plug of the present invention preferably has a connection means comprising, for example, a standard external thread. The thread can be, for example, right-hand xc2xexe2x80x3 with 16 threads per inch. Other methods for holding the plug in the chuck may also include internal threads and key grooves or like interlocking or interfitting connections.
Due to the nature of the Friction Pull Plug Welding process, xe2x80x9ccoldxe2x80x9d plug material is always being pulled into a xe2x80x9chotxe2x80x9d interface, with the plug shaft being the first location of the plug to be heated. This situation poses a problem to ensuring complete plug/plate interfacial bonding at the top of the weld, the last location of bonding during the welding process. Since the lack of bonding defect at the topside of the weld is easily detected through dye-penetrant non-destructive inspection, it has been thoroughly characterized and analyzed. The problem of bonding this last interfacial location is quite complex, involving a combination of both loading and heat flow.
Experimental data has shown that the mass of plug heat sink remaining above the top of the plate surface after a weld is completed (the plug heat sink) affects the bonding at the plug top. A minimized heat sink ensures complete bonding of the plug to the plate at the plug top. However, with a minimal heat sink three major problems can arise, the entire plug could be pulled through the plate hole, the central portion of the plug could be separated along grain boundaries, or the plug top hat can be separated from the body. The Chamfered Heat Sink Pull Plug Design allows for complete bonding along the inside skin line (ISL) interface through an outside diameter minimal mass heat sink, while maintaining enough central mass in the plug to prevent plug pull through, central separation, and plug top hat separation.
A larger plug mass acting as a heat sink at the top of the weld has deleterious affects on the bonding of the plug top. Thermodynamically, plug mass acts to conduct heat away from the interface, while the atmosphere insulates the interface, increasing the heating capabilities of the welding process. With less plug mass, or heat sink, left on top of the plate surface after the completion of a weld, the heat produced by the weld process is not conducted away from the interface as rapidly.
A chamfered heat sink pull plug design allows for complete bonding along the ISL interface through an outside diameter minimal mass heat sink, while maintaining enough central mass in the plug to pull plug failure. Experimental data has shown that complete bonding at the plug top occurs when the outside radius of the chamfered heat sink plug (LTTA (ledge top to top after) in Heating Displacement Model) is 0.00 to 0.075 inches above the plate surface at weld completion. The plug typically has a minimized heat sink band 0.050 to 0.150 inches wide around the outside diameter. From this band, the plug transitions to a 0.050 to 0.150 inch thick central chamfered heat sink.